1. Field of the Invention
The present invention relates to a thermal head, a printer, and a thermal head manufacturing method.
2. Description of the Related Art
There have been conventionally known a thermal head which is used in a thermal printer often installed to a portable information equipment terminal typified by a compact hand-held terminal, and which is used to perform printing on a thermal recording medium based on printing data with the aid of selective driving of a plurality of heating elements (for example, see Patent Document JP 2007-320197 A).
In terms of an increase in efficiency of the thermal head, there is a method of forming a heat insulating layer below a heating portion of a heating resistor. By formation of the heat insulating layer below the heating portion, of an amount of heat generated in the heating resistor, an amount of upper-transferred heat which is transferred to an abrasion resistance layer formed above the heating portion becomes larger than an amount of lower-transferred heat which is transferred to a heat storage layer formed below the heating portion, and hence energy efficiency required during printing can be sufficiently obtained. In the thermal head described in Patent Document JP 2007-320197 A, a hollow portion is provided between an upper substrate and a lower substrate which are integrated, and this hollow portion functions as a hollow heat insulating layer. Thus, the amount of upper-transferred heat becomes larger than the amount of lower-transferred heat, and the energy efficiency is increased.
Further, in a printer in which a thermal head is installed, thermal paper is pressed, with a predetermined pressing force, against a head portion formed above the heating portion by a platen roller. Therefore, the thermal head is required to have heating efficiency for improving printing quality as described above, and required to have strength for withstanding the pressing force of the platen roller.
Increasing the thickness dimension of the hollow portion by making the heat storage layer, which supports the heating resistors, thin enhances the heat insulation performance and improves the heating efficiency proportionally. On the other hand, as the thickness of the heat storage layer is reduced, the strength for supporting the heating resistors is reduced. The heat storage layer should therefore be set to a desired thickness in order to improve the heating efficiency and strength of the thermal head.
Patent Document 1 describes a thermal head manufacturing method in which the hollow portion is formed by forming a gap within a convex portion, which is formed on one surface of the upper substrate, and closing up the gap through fusion-bonding of the flat lower substrate to the other surface of the upper substrate. There is a possibility with this manufacturing method that, if a load is applied upon bonding to a surface of the upper substrate softened by fusion, the convex portion of the upper substrate is deformed to cause fluctuations in the thickness of the upper substrate, namely, the heat storage layer. The resultant problem is that stable manufacture of a thermal head improved in heating efficiency and strength is difficult.